1. Field of the Invention
The present invention concerns a spectral rejection device for forming an image on an optical sensor, the device including a system for forming a beam of parallel light rays coming from an observed scene, and an optical system for focusing the beam onto the optical sensor.
2. Brief Description of the Prior Art
It is known in the prior art to detect or identify a particular characteristic of a light beam coming from an observed scene by comparing images of the beam formed on an optical sensor respectively in the absence of a filter and after passing through a filter, stopping a narrow band of wavelengths that is particular to the characteristic looked for, for example the presence of a polluting or toxic gas in the observed area.
The U. S. patent to Krasno No. 2,721,259 proposes an information transmission system in which a light beam emitted by a flash lamp is dispersed spectrally before passing over an obstacle that can be moved in the direction of spectral dispersion of the light beam to modulate the beam spectrally by movement of the obstacle controlled by the information to be transmitted.,
The light beam is then combined spectrally in order to be transmitted and is then dispersed spectrally before impinging on a receiver, such as a photoelectric cell, whose output signal is modulated in a manner corresponding to the movement of the obstacle in the emitted light beam. The above prior art system does not constitute an optical filter for stopping a predetermined narrow band of wavelengths in a light beam coming from an observed scene and cannot be used for that purpose.
It can be difficult to make optical filters having substantially zero transmission over a relatively very narrow band of wavelengths and use of such filters is subject to a number of drawbacks:
the filters generally modify the sensitivity and the operating point of the optical sensors,
the band of wavelengths stopped by the filter cannot be modified or modulated continuously,
rotating filters generally saturate the sensor and significantly and permanently modify its operating point when the filter support passes in front of the sensor,
the presence of a filter on the optical path induces a geometrical offset between the images obtained through the filter and those obtained without the filter,
the image obtained through a rotating filter is not sharp, etc.
An object of the present invention is to avoid the above drawbacks.
Accordingly, it is a primary object of the present invention to provide a spectral rejection device for forming an image on an optical sensor, including a system for forming a beam of parallel light rays coming from an observed scene, a system for spectrally dispersing the beam in at least one direction along which the wavelengths of the dispersed light rays vary substantially continuously, at least one mask disposed on the path of dispersed rays to intercept at least one band of wavelengths, a system for positioning and/or moving the mask substantially in the direction previously cited, a system for combining dispersed rays into a beam of parallel rays, and a system for focusing the combined beam onto the sensor, characterized in that the mask is dimensioned to intercept a narrow band of wavelengths without modifying the sensitivity or the operating point of the sensor and without significantly modifying the transmission of the device when it is positioned or moved in the direction previously cited.
Generally speaking, the expression xe2x80x9cnarrow band of wavelengthsxe2x80x9d means that the band of wavelengths intercepted by the mask is relatively very narrow relative to the spectral width of the light beam and/or relative to the spectral operating band of the optical sensor.
The device in accordance with the invention has none of the drawbacks of the filters used in the prior art technique:
it is easily inserted into the optics of the sensor,
it does not modify the sensitivity or the operating point of the sensor,
the transmission of the device does not depend on the band of wavelengths stopped,
the wavelength or wavelengths stopped within the spectral operating band of the sensor can be modulated continuously and at a high rate, for example that of a video camera,
the variation of the wavelength or wavelengths stopped does not modify the operating point of the sensor,
the presence of the device does not degrade the quality of the image obtained.
In various embodiments of the invention:
the system for spectrally dispersing the beam includes a prism and a lens placed between the prism and the mask, the latter lying in the focal plane of the lens,
the combiner system includes a second lens and a second prism, for example respectively identical to the lens and the prism of the disperser system, the mask lying in the object focal plane of the second lens,
the disperser system includes a plane or concave grating and the combiner system includes a concave mirror receiving rays dispersed by the grating and reflecting them toward a collimator lens, the mask being placed on the path of rays dispersed by the grating or on the path of rays reflected by the concave mirror,
the disperser system includes a concave grating associated with a plane mirror that receives rays dispersed by the concave grating and reflects them toward the mask and the combiner system,
the combiner system includes a plane mirror and a concave grating, the plane mirror receiving rays leaving the disperser system and reflecting them toward the concave mirror, which forms them into a beam of parallel rays.
the disperser system includes two identical components, such as prisms or gratings, disposed one after the other and reversed one relative to the other to provide at the output of the second component a beam of parallel monochromatic rays,
the combiner system includes two identical components, such as prisms or gratings, disposed one after the other and reversed one relative to the other, the first component receiving rays spectrally separated by the disperser system and directing them toward the second component which combines them into a beam of parallel polychromatic rays,
the combiner system for combining the dispersed rays and the focusing system for the sensor are one and the same.
Advantageously, the mask is moved periodically at a rate synchronized to the operation of a video camera.
The mask and its positioning and displacement systems can advantageously consist of a galvanometer, the pointer of which forms the mask. The mask can instead be a pointer mounted on a carriage that is motorized or driven by other appropriate means.
In one particular embodiment of the invention, the optical sensor includes Time Delay Integration (TDI) sensors associated with data processing means for simultaneous multispectrum analysis of the captured beam. A data processing system associated with the optical sensor is used to subtract from each other the output signals of the TDI sensors acquired simultaneously or for subtracting from each other output signals of the optical sensor acquired successively for slightly different positions of the mask and to differentiate with respect to the wavelength the luminous flux received by said optical sensor.
Generally speaking, the invention enables modulation at will of a narrow spectral rejection in a light beam received by an optical device that forms an image of it on a sensor. The invention also rejects one or more fixed wavelengths in a luminous flux received by a sensor. Its applications are therefore numerous and varied.